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G. B. Hansen (Planetary Science Institute)
The optical constants of “typical” Martian dust are needed for modeling dust in the polar caps and for investigating surface spectral properties under an aerosol layer. I have derived real and imaginary indices of refraction of Martian dust at infrared wavelengths >7.5 \mum (< 1350 cm-1) by simulating the spectrum of a scattering and emitting aerosol-laden atmosphere and comparing it to spectra returned by the Mariner 9 infrared spectrometer (IRIS). One smoothed spectral average from late in the mission, when the 1971 global dust storm was well into its decay, was analyzed in detail. It was selected over a fairly dust covered surface to minimize the surface emissivity contrast and so a blackbody surface could be used in the initial models. The input optical constants were adjusted over four 9-\mum band-maximum imaginary refractive indices from 0.4 to 1.25, and seven particle size distribution functions with effective radii from 0.7 to 2.8 \mum (as defined by previous estimations). The average spectrum could be fitted equally well using any 9-\mum band strength or particle size distribution. Few of these alternatives can be excluded using broad constraints, but one can invoke plausibility arguments that narrow the range of possible candidates greatly. So a median model with realistic properties was selected from the alternatives and refined by modeling with an underlying dust surface with identical optical properties. These refined constants are then compared to several IRIS spectral averages over dusty regions to determine the uncertainies in the constants from temporal and spatial variations and other factors. Models using the new optical properties fit the aerosol spectra much better than previous attempts that used the optical properties of terrestrial simulants, and have also been used successfully for spectral modeling of CO2 ice and dust mixtures in the polar caps.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.